Missing Sydney boy found 'smiling' in stairwell, 100 metres from police station

After three days of desperate searching, a shoeless but smiling 12-year-old Hussein Al Mansoory was found alone in a stairwell of a medical centre in Sydney's south-west.

A member of staff called police after stumbling across Hussein at 12.40pm today, in a building less than 100 metres from the Auburn Police Station.

Fears were growing for Hussein's safety as the search dragged on, because he is non-verbal, autistic and lives with Down syndrome.

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The young boy was described as safe and well, and his family "ecstatic", but he was taken to hospital as a precautionary measure.

"He smiled. He was sitting up," Superintendent Simon Glasser said, hours after he had declared the search was at a critical stage, as it entered day three.

"His family were told immediately that we had found him," Glasser said.

"They were ecstatic and attended the scene straightaway."

Hussein's family enveloped him with joyous hugs, Glasser said.

"It's a great result."

Police are still working out how Hussein had gained access to the building, and when.

The stairwell appeared to be an internal fire escape, Glasser said, and he was unsure how frequently it was used.

"He was inside the stairwell. I don't have the exact information of the entry and whether the doors were locked or unlocked. But that's where he was."

A huge police team and "hundreds" of specialist SES searchers and volunteers were involved in a search which covered the entirety of Auburn.

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Hussein's multiple conditions made him "very vulnerable", particularly given soaring temperatures over the weekend, and complicated the task of finding him.

"This is as high risk as it gets for us with a missing person," Glasser said today, before Hussein was located.

Police choppers flew above Auburn, broadcasting messages from loudspeakers, hoping to reach Hussein who it was thought could be hiding out in the suburb's many commercial and warehouse environments or on private land.

SES Commander Jamie Newman described a "highly complex" search in "a built-up mini CBD with some high-rise buildings (and) some areas inaccessible to us or the volunteers searching".

Hussein's family had endured a torturous weekend after the young boy ran from Auburn Memorial Park on Saturday morning, triggering a vast search operation.

He was later seen about 9pm that night.

That was the last of two confirmed sightings on Saturday, and police had received multiple possible sightings on Sunday.

As temperatures in Sydney hovered around 30 degrees over the weekend, Hussein's family and police had worried how the boy would cope.

Early this morning Glasser said he hoped Hussein had managed to find some food.